1. Field of the Invention
The present invention relates to a process for producing an information recording medium used as a recording medium of an information processing apparatus and also a process for producing a substrate employed for the medium.
2. Description of the Related Art
One of such information recording mediums is a magnetic disk. The magnetic disk is produced by forming a thin film such as a magnetic layer on a substrate and as the substrate for it, an aluminum or glass substrate has been employed. Reflecting the recent pursuit of high recording densification, a glass substrate which makes it possible to decrease a distance between a magnetic head and a magnetic. recording medium has come to be more prevalent compared with an aluminum substrate.
The glass substrate showing such an increasing tendency is, in general, subjected to chemical reinforcement for heightening its strength so that it can endure the impact upon mounting on a magnetic disk driver. The surface of the glass substrate is polished with high accuracy so that the flying height of the magnetic head can be lowered to the utmost, whereby the high recording densification is actualized.
As well as the improvement in the glass substrate, a magnetic head has been changed from a thin-film head to a magnetoresistive type head (MR head) to meet the tendency to high recording densification.
As described above, high evenness on the surface of a magnetic disk is indispensable for the flying height reduction to heighten the recording density. In addition, when an MR head is employed, high flatness on the surface of a magnetic recording medium is also required in order to prevent the generation of TA (thermal asperity). The term xe2x80x9cthermal asperityxe2x80x9d as used herein means a phenomenon that a projection existing on the surface of a magnetic disk affects an MR head to generate heat, thereby causing fluctuations in the resistance of the head and causing a malfunction in the electromagnetic conversion.
There is an increasing demand for a magnetic disk having high evenness for both the lowering of flying height and prevention of occurrence of thermal asperity. A substrate having a highly even surface is eventually required for a magnetic disk having a surface of high evenness, but the requirement for heightening of the recording density exceeds the level which can be materialized by only polishing the surface of the substrate with high accuracy. In other words, it is impossible to attain high evenness even by polishing with high accuracy if foreign matters adhere onto the substrate. The removal of foreign matters has been carried out conventionally but the conventional tolerance of foreign matters on the substrate is now regarded as inadequate for the recent tendency to heighten the recent densification level.
Examples of such foreign matters include considerably fine iron powders, stainless pieces, glass chips and organic substance which cannot be removed by ordinary washing. When a thin film such as magnetic film is stacked on a glass substrate without removing the particles such as iron powders from the glass substrate, a protrusion is formed on the surface of a magnetic disk, which becomes a cause for disturbing the lowering in the flying height or prevention of thermal asperity.
An object of the present invention is to prevent the adhesion of particles such as fine iron powders to a glass substrate. Another object of the present invention is to produce an information recording medium in a high yield by using a glass substrate from which foreign matters to be a cause for under-layer defects have been removed.
The present inventors have conducted an extensive investigation on the cause of the adhesion of the particles such as fine iron powders to a glass substrate. As a result, it has been found that a metal such as iron powders or metal oxide generated from manufacturing facilities or buildings is floating in an atmosphere surrounding a chemical reinforcement apparatus and such a floating substance is mixed in a chemical reinforcement solution and adheres to the glass substrate.
In one aspect of the present invention, there is thus provided a process for producing a glass substrate for an information recording medium, which comprises a step of polishing a principal surface of a glass substrate and a step of chemically reinforcing said glass substrate (said step may hereinafter be called xe2x80x9cchemical reinforcement stepxe2x80x9d) by bringing it into contact with a chemical reinforcement liquid to substitute a portion of ions contained in said glass substrate by ions having a diameter larger than the former ions in said chemical reinforcement liquid, wherein at least one of the steps subsequent to a final polishing step, that is, washing step, said chemical reinforcement step, step of washing off said chemical reinforcement liquid, drying step, inspection step, and a step of packing said glass substrate or filling the same in a container is effected in an atmosphere where clean filtered air is circulated so as to prevent the adhesion of particles to said glass substrate.
In a second aspect of the present invention, there is also provided a process for producing a glass substrate for an information recording medium, which comprises a step of polishing a principal surface of a glass substrate; and a step of chemically reinforcing said glass substrate by bringing it into contact with a chemical reinforcement liquid to substitute a portion of ions contained in said glass substrate by ions having a diameter larger than the former ions in said chemical reinforcement liquid, thereby reinforcing said glass substrate, wherein at least one of the steps subsequent to a final polishing step, that is, a washing step, said chemical reinforcement step, a step of washing off said chemical reinforcement liquid, a drying step, an inspection step, and a step of packing said glass substrate or filling the same in a container is effected under an environment whose air cleanliness permits the existence of particles having a size of 0.3 to 100 xcexcm in an amount not more than 1000 particles per cubic feet-meter of the air.
In a third aspect of the present invention, there is also provided a process for producing a glass substrate for an information recording medium as described in the first aspect, wherein the clean filtered air has cleanliness permitting the existence of particles having a size of 0.3 to 100 xcexcm in an amount not more than 1000 particles per cubic feet-meter of the air.
In a fourth aspect of the present invention, there is also provided a process for producing a glass substrate for an information recording medium as described in the first or second aspect, wherein the air has cleanliness permitting the existence of particles in an amount not more than 100 particles per cubic feetxc2x7meter of the air.
In a fifth aspect of the present invention, there is also provided a process for producing a glass substrate for an information recording medium as described in the first or second aspect, wherein the air has cleanliness permitting the existence of particles in an amount not more than 50 cubic feetxc2x7meter of the air.
In a sixth aspect of the present invention, there is also provided a process for producing a glass substrate for an information recording medium as described in the first or second aspect, wherein the washing and chemical reinforcement of the glass substrate subsequent to the final polishing step are effected under an environment whose air cleanliness permits the existence of particles in a prescribed amount in the air.
Examples of the particles to be removed in the present invention include metal pieces such as fine iron powders, metal oxide pieces, glass chips, oil mist, silicon dust, fibers and organic substance. Particles large enough to cause inferiority during the use of an information recording medium or in the production step are removed. The size of the particles to be removed preferably ranges from 0.3 micron to 100 microns, but control to a size within a range of from 1 micron to 100 microns also brings about effects. Even the control of the size to 10 to 100 microns does not cause problems in the practical use.
The steps subsequent to the final polishing such as washing, chemical reinforcement treatment, washing-off of the chemical reinforcement solution, drying, inspection, or packing of the glass substrate may be carried out after covering, with a dust-tight cloth, a part of the manufacturing facilities or buildings which possibly generate dust; or may be carried out in a room having cleanliness controlled to a predetermined degree by a clean booth method or clean room method, in order to reduce the particles, thereby heightening cleanliness.
The step of reducing particles to heighten the cleanliness may be carried out at least one of the steps subsequent to the final polishing such as washing, chemical reinforcement treatment, washing-off of the chemical reinforcement solution, drying, inspection and packing of the glass substrate, but it is desired to carry out this step for heightening the cleanliness in all of these steps. It is particularly effective to reduce the particles, thereby improving cleanliness during the final polishing step to the chemical reinforcement step, because when particles have adhered to the glass substrate during the chemical reinforcement step, a portion of the glass substrate on which the particles exist is not chemically reinforced or particles on the glass substrate firmly adhere to the glass substrate due to the chemically reinforced salt and cannot be removed easily by the subsequent washing step. Accordingly, it is markedly important to maintain the cleanliness of the glass substrate during the chemical reinforcement step or the step therebefore.
The present inventors have carried out a further extensive investigation on the cause of the adhesion of the fine iron powders to a glass substrate. As a result, it has been found that when the glass substrate is immersed in a chemical reinforcement tank containing a chemical reinforcement solution therein or a chemical reinforcement solution, a metal such as iron powders or a metal oxide adheres to the glass substrate directly or through the chemical reinforcement solution from a retaining means for retaining the glass substrate. It has also been found that since the chemical reinforcement treatment is effected at a high temperature (ex. 350 to 450xc2x0 C.), a chemically reinforced glass substrate having a clean surface cannot be obtained unless a specific stainless alloy is used.
In a seventh aspect of the present invention, there is therefore provided a process for producing a glass substrate for an information recording medium, which comprises a step of chemically reinforcing a glass substrate, which has been retained by a retaining means, by bringing its into contact with a chemical reinforcement liquid to substitute a portion of ions contained in said glass substrate by ions having a diameter larger than the former ions in said chemical reinforcement liquid, wherein at least a wall surface of a chemical reinforcement tank containing said chemical reinforcement liquid therein and/or said retaining means for retaining said glass substrate which is in contact with said chemical reinforcement liquid is formed of a stainless alloy having corrosion resistance in a high-temperature region as high as the heating temperature of said chemical reinforcement liquid.
In an eighth aspect of the present invention, there is also provided a process for producing a glass substrate for an information recording medium as described in the seventh aspect, wherein said stainless alloy is a martensitic or austenitic stainless alloy.
Various shapes of retaining means can be considered as those for the glass substrate. In short, preferred is a retaining means shaped to make it possible to bring the glass substrate into contact with the chemical reinforcement solution under a predetermined condition and not to cause liquid sagging.
It is preferred to form both the chemical reinforcement tank and retaining means for the glass substrate from a martensitic or austenitic stainless alloy, but it is also possible to form either of them from such an alloy. Moreover, it is preferred to form the whole portion of the chemical reinforcement tank or of the retaining means for the glass substrate from a martensitic or austenitic stainless alloy, but it is possible to form only a portion which is brought into contact with the chemical reinforcement solution from such an alloy.
The martensitic or austenitic stainless alloy has excellent corrosion resistance in a high temperature region so that it prevents the generation of dust from metallic pieces. The chemical reinforcement solution is presumed to be heated at 200 to 500xc2x0 C.
The present inventors have carried out a still further investigation on the cause of the adhesion of fine iron powders, glass chips or stainless pieces to a glass substrate. As a result, it has been found that metal pieces such as iron powders generated from various manufacturing facilities or buildings installed around the chemical reinforcement apparatus are mixed in the chemical reinforcement liquid and they are adhered to a glass substrate during the chemical reinforcement of the glass substrate.
In a ninth aspect of the present invention, there is therefore provided a process for producing a glass substrate for an information recording medium, which comprises a step of chemically reinforcing a glass substrate by bringing it into contact with a chemical reinforcement liquid to substitute a portion of ions contained in said glass substrate by ions having a diameter larger than the former ions in said chemical reinforcement liquid, wherein a means for trapping fine particles existing in said chemical reinforcement liquid is provided.
In a tenth aspect of the present invention, there is also provided a process for producing a glass substrate for an information recording medium as described in the ninth aspect, wherein said means for trapping fine particles is a filter for filtering said chemical reinforcement liquid circulated.
In an eleventh aspect of the present invention, there is also provided a process for producing a glass substrate for an information recording medium as described in the ninth aspect, wherein said fine particles are fine iron powders and said trapping means is a magnet disposed to be in contact with said chemical reinforcement liquid.
As a chemical reinforcement method of the present invention, preferred is low-temperature type chemical reinforcement in which an ion exchange is performed in a region not exceeding the glass transition point. Examples of an alkali fusion salt used as a chemical reinforcement solution include potassium nitrate, sodium nitrate, and a mixture thereof.
Examples of the glass substrate usable in the present invention include aluminosilicate glass, soda-lime glass and crystallized glass.
As particles, metal pieces such as iron powders or stainless, metal oxides, glass chips and organic substance can be given as examples. The removal of such particles having a particle size of several microns or greater is effective for the process of the present invention.
Examples of the trapping means of the fine particles in the present invention include a filter. Any filters that can supply the filtered chemical reinforcement liquid can be employed. For example, a micro-sieve (a wire mesh having pores made by etching) can be used. In this case, the chemical reinforcement solution is heated at a high temperature so that the use of a martensitic or austenitic stainless alloy having excellent corrosion resistance is preferred.
As another means for trapping particles, a magnet or the like may be disposed so as to be in contact with the chemical reinforcement solution, in the case where the particles are fine iron powders.
The trapping means may be fixed or set movable.
The present inventors have carried out a further extensive investigation. As a result, it has been found that by washing the glass substrate with hydrochloric acid, foreign matters can be satisfactorily removed, particularly, iron contaminants dissolved therein can be removed effectively.
In the twentieth aspect of the present invention, there is therefore provided a process for producing a glass substrate for an information recording medium, said glass substrate having a principal surface subjected to precision polishing, which comprises a step of washing said glass substrate with hydrochloric acid.
In the thirteenth aspect of the present invention, there is also provided a process for producing a glass substrate for an information recording medium as described in the twentieth aspect, which comprises a step of chemically reinforcing a glass substrate by substituting a portion of ions contained in said glass substrate by ions having a diameter larger than the former ions, wherein said glass substrate is washed with hydrochloric acid as a pre-treatment or post-treatment step of the chemical reinforcement step.
In the present invention, by washing with hydrochloric acid, particularly iron contaminants dissolved therein can be removed effectively and also other foreign matters, for example, nickel, stainless, chromium, oxides or organic substance thereof can be removed. Washing with hydrochloric acid may be carried out between any two steps for the production of a glass substrate for an information recording medium but it is effectively carried out in one step or plural steps between the polishing step and packing step of the glass substrate completed. In the process for producing a glass substrate for an information recording medium including a chemical reinforcement step, when the step of washing with hydrochloric acid is carried out prior to the chemical reinforcement step, remaining of an unreinforced portion caused by foreign matters on the surface of the glass substrate can be prevented effectively. Alternatively, it is also possible to carry out the chemical reinforcement step and the step of washing off the fusion salt or the like in a clean booth or by the like method under an atmosphere where the air of a high cleanliness has been circulated and to carry out the step of washing with hydrochloric acid prior to packing of the glass substrate.
It is preferred to wash the whole portion of the glass substrate with hydrochloric acid but, the principal surface and the outer and inner peripheral sides of the glass substrate may be selectively washed in order to prevent under-layer defects and in order to prevent the transfer of the iron contaminants or the like, which has been adhered on the end face, to the principal surface, respectively. Examples of the washing method include dipping of a glass substrate in hydrochloric acid and spraying hydrochloric acid onto a glass substrate. Effects of washing with hydrochloric acid can be enhanced by applying supersonic waves during the washing with hydrochloric acid; or by scrub washing during or after washing with hydrochloric acid. As hydrochloric acid, 1-12N (N:Normality) dilute hydrochloric acid or concentrated hydrochloric acid is preferred. The temperature of hydrochloric acid is adjustable in the range of 30xc2x0 C. to 60xc2x0 C. as appropriate. The processing time of hydrochloric washing is adjustable in the range of 1 minute to 20 minutes as appropriate.
Specific examples of the glass substrate for an information recording medium include a glass substrate for a magnetic disk.
In the fourteenth aspect of the present invention, there is therefore provided a process for producing a glass substrate for an information recording medium as described in any one of the first, second, seventh, ninth and twelfth aspects, wherein said glass substrate for an information recording medium is a glass substrate for a magnetic disk.
In the fifteenth aspect of the present invention, there is also provided a process for producing a glass substrate for an information recording medium as described in the fourteenth aspect, wherein said glass substrate for a magnetic disk is a glass substrate for a magnetic disk for a magnetoresistive type head.
In the sixteenth aspect of the present invention, there is also provided a process for producing an information recording medium, which comprises forming at least a recording layer on a glass substrate obtained by the process for producing a glass substrate for an information recording medium as described in any one of the first, second, seventh, ninth and twentieth aspects.
In a seventeenth aspect of the present invention, there is also provided a process for producing an information recording medium as described in the sixteenth aspect, wherein said recording layer is a magnetic layer.
In the present invention, no particular limitation is imposed on the kind, size, thickness or the like of a glass substrate. Examples of the material quality of the glass substrate include aluminosilicate glass, soda-lime glass, soda aluminosilicate glass, aluminoborosilicate glass, borosilicate glass, quartz glass, chain silicate glass and glass ceramics such as crystallized glass.
As an aluminosilicate glass, preferred is a glass for chemical reinforcement which contains as principal components 62 to 75 wt. % of SiO2, 5 to 15 wt. % of Al2O3, 4 to 10 wt. % of Li2O, 4 to 12 wt. % of Na2O and 5.5 to 15 wt. % of ZrO2; and wherein the weight ratio of Na2O to ZrO2 is 0.5 to 2.0 and that of Al2O3 to ZrO2 is 0.4 to 2.5.
It is preferred to use a glass for chemical reinforcement containing 57 to 74% of SiO2, 0 to 2.8% of ZrO2, 3 to 15% of Al2O3, 7 to 16% of Li2O and 4 to 14% of Na2O, each in terms of mole %, in order to avoid the appearance of protrusions on the surface of the glass substrate which results from the undissolved substance of ZrO2.
The aluminosilicate glass or the like having such a composition has increased bending strength, a deepened compression stress layer and improved Knoop hardness.
In the present invention, for the improvement of impact resistance and vibration resistance, chemical reinforcement treatment is applied to the surface of a glass substrate by the low-temperature ion exchange method.
No particular limitation is imposed on the chemical reinforcement method insofar as it is the chemical reinforcement method well known in the art. Preferred is low-temperature type chemical reinforcement in which ion exchange is effected in a temperature range not exceeding the glass transition point. Examples of the alkali fusion salt used for chemical reinforcement include potassium nitrate and sodium nitrate, and a mixture thereof.
The glass substrate for an information recording medium according to the process of the present invention can be used as a glass substrate for a magnetic recording medium, a glass substrate for an optical magnetic disk and a glass substrate for an electro-optical disk.
In particular, the glass substrate can be used suitably as a magnetic disk substrate for a magnetoresistive type head (including large-sized magnetoresistive type head) which carries out recording and reproduction and it can also be used suitably for a process for producing an information recording medium by using the-substrate.
The magnetic recording medium of the present invention will hereinafter be described.
The magnetic recording medium of the present invention has at least a magnetic layer formed on the glass substrate for a magnetic recording medium according to the present invention.
Particles causing thermal asperity or head crash are not generated in the present invention so that protrusions, which will otherwise be formed by the particles, do not appear on the principal surface of the glass substrate at the time when a magnetic recording medium is produced by forming a magnetic layer on the glass substrate, whereby the prevention of a head crash on a still higher level can be attained. Particularly for a magnetic recording medium which carries out reproduction by a magnetoresistive type head, absence of protrusions makes it possible to fully exhibit the function of the magnetoresistive type head. Also for a CoPt type magnetic recording medium which is suitably used for a magnetoresistive type head, absence of protrusions makes it possible to fully exhibit the performance of the head.
Also on the recording or reproducing surface of the magnetic recording medium, no protrusion causing thermal asperity is formed, whereby the prevention of head crash on a still higher level can be effected.
Furthermore, since no particles which will cause thermal asperity are generated, neither defects in a film such a magnetic layer nor errors occur.
The magnetic recording medium is fabricated by stacking a primary layer, magnetic layer, protective layer and lubricating layer successively on a glass substrate for a magnetic disk which has a predetermined evenness and surface roughness and has a surface subjected to chemical reinforcement as needed.
The primary layer of the magnetic recording medium according to the present invention may be selected, depending on the magnetic layer stacked thereon.
Examples of the primary layer include those composed of at least one material selected from nonmagnetic metals such as Cr, Mo, Ta, Ti, W, V, B and Al. When the magnetic layer is composed mainly of Co, the primary layer is preferably formed of a single substance or alloy of Cr from the viewpoint of the improvement in the magnetic characteristics. The primary layer is not limited to a single layer but may be formed of plural layers of the-same material or different materials. Examples of the multi-layered primary layer include Cr/Cr, Cr/CrMo, Cr/CrV, CrV/CrV, Al/Cr/CrMo, Al/Cr/Cr, Al/Cr/CrV and Al/CrV/CrV.
No particular limitation is imposed on the material of the magnetic layer in the magnetic recording medium of the present invention.
Examples of the magnetic layer include magnetic layers composed mainly of Co such as CoPt, CoCr, CoNi, CoNiCr, CoCrTa, CoPtCr, CoNiPt, CoNiCrPt, CoNiCrTa, CoCrTaPt and CoCrPtSiO. Alternatively, the magnetic layer may have a multi-layered structure (ex. CoPtCr/CrMo/CoPtCr, CoCrTaPt/CrMo/CoCrTaPt) which aims at a noise reduction by dividing the magnetic film by a nonmagnetic film (ex. Cr, CrMo, CrV).
Examples of the magnetic layer for a magnetoresistive type head (MR head) or giant magnetoresistive type head (GMR head) include Co alloys in which an impurity element selected from Y, Si, rare earth elements, Hf, Ge, Sn and Zn or an oxide of such an impurity element has been incorporated.
The magnetic layer may have a granular structure in which magnetic particles such as Fe, Co, FeCo or CoNiPt, have been dispersed in a non-magnetic layer formed of ferrite, ironxe2x80x94rare earth element, SiO2 or BN. In addition, the recording system of the magnetic layer may be either an internal recording or a vertical recording system.
No particular limitation is imposed on the protective layer in the magnetic recording medium of the present invention.
Examples of the protective layer include Cr film, Cr alloy film, carbon film, zirconium film and silica film. The protective layer can be formed continuously with a primary layer and magnetic layer by an in-line sputtering apparatus. The protective layer may be a single layer or a multi-layer formed of the same material or different materials.
On the above-described protective layer or instead of the above-described protective layer, another protective layer may be formed in the present invention. For example, it is possible to disperse colloidal silica particles in a diluted solution of tetraalkoxysilane in an alcohol solvent and apply the resulting dispersion to a Cr film, followed by calcination, whereby a silicon oxide (SiO2) film is formed instead of the above-described protective layer.
No particular limitation is imposed on the lubricating layer in the magnetic recording medium of the present invention.
The lubricating layer is formed, for example, by diluting perfluoropolyether (PFPE), which is a liquid lubricant, with a fluorene base solvent and applying the resulting diluted solution onto the surface of the medium by the dipping method, spin coating method or spraying method and optionally heating the coated surface.